Surgery – Instruments – Orthopedic instrumentation
Reexamination Certificate
2001-05-14
2004-10-26
Casler, Brian L. (Department: 3763)
Surgery
Instruments
Orthopedic instrumentation
C606S075000
Reexamination Certificate
active
06808526
ABSTRACT:
The invention relates to an osteosynthesis screw, in particular for application in translaminar vertebral-screw fixation, with a shaft, at least partially comprising a thread, and a head formed at least at one of its ends and comprising a notch arrangement for a tool.
Translaminar screw fixation on the lumbar vertebrae in the context of spinal fusion (spondylosyndesis) and operative treatment of injuries has been used for almost twenty years. The principle of translaminar screw fixation consists of the use of osteosynthesis screws to block the zygapophyseal joints, to prevent any possible movement between vertebrae; with the resulting immobilisation of the respective section of the lumbar spine safeguarding consolidation of the spondylosyndesis or healing of the injury. The screws enter on one side of the spinous process of the bone, extend through the mutual lamina, traverse the zygapophyseal joints and end up in the base of the transverse process of the lower vertebra. The free end of the screw is then located in the ventrocaudal delimitation of the transverse process, but it must not protrude at that location.
There is only limited space available for placement of translaminar screws in the dorsal structures of the vertebrae. For anatomical reasons the tolerances are very limited in regard to the most favourable screw position both from an anatomical and a mechanical perspective. Even minor deviation from the ideal position brings about a rapid decrease in effectiveness of the respective screw. For this reason, the use of a special target device is recommended for placement of the screws.
Due to the fact that in each vertebra there is only one ideal position for each of the two translaminar screws, the problem arises in the region of the spinous process of the upper vertebra, i.e. where the screws are inserted in the bone, where they cross over, in that it is not possible to position both screws ideally. Consequently, the position of the second screw must already be considered at the time of setting the first screw. Otherwise, only a very unfavourable placement option may remain for the second screw. In particular in the case of the fifth lumbar vertebra, the positioning margin is very small.
In principle the screws act like pins that block the facet joint of the spine. Because smooth pins would slide in the bone and other alternatives have not been available, normal osteosynthesis screws with a continuous thread have been used so far. This is however associated with several disadvantages: in the case of harder bones, the thread needs to be pre-cut, a procedure which is cumbersome and which increases the danger of neurotrauma or nerve damage. Due to the thread which is of no use as far as mechanical strength is concerned, the outer diameter of the screw is unnecessarily large. During the healing process which can take several months, in the region of the zygapophyseal joints, a large number of forces act upon the screws and their bedding, transversely to the screw axis. Consequently, over time, this bedding can be destroyed by the sharp ridges of the thread. Furthermore, screws provide the temptation of excessive tightening. Because the thread of translaminar screws is pre-cut so as to be continuous, i.e. the thread continues in the spinous process and the subsequent lamina, excessive tightening of the screw may cause the spinous process to break off. Conventional translaminar vertebral-screw fixation is associated with a serious disadvantage in that due to the need for positioning the screws in the spinous process so that they cross in close proximity, and due to the limited space available in this position, placement of the second screw must already be considered before placing the first screw. Otherwise it may happen that only one screw is favourably placed from an anatomical and mechanical point of view. Furthermore, the spinous process is weakened by two screw channels crossing in close proximity. This can have a negative effect if an additional loadbearing element, e.g. a bone chip, is be jammed between the spinous processes. The protruding head of conventional screws used so far, prevents any embedded arrangement. Later on, it must be possible to remove the screws again.
It is the object of the present invention to form an osteosynthesis screw of the type mentioned in the introduction, that can be screwed-in in a way that is gentle to the bone, that provides security against axial movement and against rotational movement, and that will achieve an improvement in application techniques.
According to the invention it is proposed that the shaft has a section with a thread on the end section oriented towards the head, that the remaining section of the shaft up to the free end is without a thread, that the diameter of the head corresponds at least approximately to the outer diameter of the thread and comprises an inner notch arrangement for a tool, and that the section of the shaft with thread and/or the section of the shaft without thread is noncircular, of uniform thickness e.g. trilobular or polygonal, and/or comprises elevations and/or indentations.
By having a thread only in part of the shaft and by having a large section without a thread, with a larger core diameter, damage to the bone wall can be kept very slight. Thus the thread is provided only to prevent dislocation of the osteosynthesis screw in axial direction. When in addition the section with the thread and/or the threadless section is non-circular or similar, from the very beginning after setting the screw, positive locking between the bone material and the surface of the osteosynthesis screw is achieved by elastic adaptation of the elastically deformable bone and later by regrowing bone. In this way from the very beginning and increasingly thereafter, the new screw provides security against automatic unscrewing due to repetitive load changes, a problem occurring with conventional osteosynthesis screws. Furthermore, as a result of the special configuration of the head, the osteosynthesis screw according to the invention can be embedded, i.e. it can interact along its entire length with the bone components to be blocked, without protruding. In this way potentially dangerous fracture situations can be avoided which may well occur if there was a screw head to rest against a bone component, during screwing-in of the screw.
Since security against rotation of the osteosynthesis screw does not require great forces, positive locking by elastic deformation and by regrowing bone tissue is perfectly adequate. This also provides the option of subsequent removal of the osteosynthesis screw, because applying some force to the osteosynthesis screw in rotational direction will undo this positive locking without any problems.
The measures according to the invention make possible two techniques of application. Where there is sufficient space, the screws can be implanted in the conventional way, i.e. with the head exposed on the surface of the spinous process and with the screws crossing over in the spinous process. In the second option, which in many cases is the more favourable technique, first a shorter screw can be inserted, with its head embedded. The second screw can then be implanted without any regard to the position of the first screw. This technique provides the advantage in that it does not require any compromises regarding the most favourable anatomical and mechanical screw position. Both screws can be placed optimally.
With such a osteosynthesis screw it is advantageous if the thread of the shaft is self-tapping and/or self-cutting.
A particular embodiment provides for the thread-free section of the shaft to be cylindrical or conical, with the section of the shaft comprising the thread being non-circular, of uniform thickness e.g. trilobular or polygonal, and/or comprising elevations and/or indentations. To provide security against rotation it is perfectly adequate if only the short section of the thread is e.g. trilobular. In addition, screwing-in the osteosynthesis screw and thus production of the thread in the
Dransfeld Clemens
Magerl Fritz
Mayer Jörg
Spirig Walter
Tognini Roger Roland
Casler Brian L.
Kontos Lina R
Sepitec Foundation
Volpe and Koenig P.C.
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